The bug
All the fish appear to be swimming on their side and flapping their tails up and down. Most likely caused by MC-129409.
Linked issues
is duplicated by
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Actually, it relates to MC-129409.
Multiplayer. I notice the sinking effect, but i was in creative and thought it was a new feature for that mode.
Seems to be resolved....
Are you on a singleplayer world, or on a multiplayer one?
If you're on multiplayer, then this is a duplicate of MC-129409, as that's an issue stating that water acts like air in multiplayer.